Until recently, the resin systems used in coatings, e.g., for architectural paints or wood finishes, were delivered in organic solvent systems. Since organic solvent systems release volatile organic compounds into the air, it is desirable to replace organic solvent systems with waterborne systems. The challenge is to develop waterborne systems which can provide performance benefits, such as high gloss, water resistance, chemical/solvent resistance, and abrasion resistance, which are comparable to organic solvent systems.
It is known that aqueous acrylic dispersions are useful as binders for waterborne coatings. Aqueous acrylic dispersions, stabilized by surfactants, are commonly employed in paints, inks, varnishes and adhesives. In paints, the choice of surfactants for stabilization of the polymeric dispersion in water can provide small particle size latices, resulting in high gloss coatings.
It is also known that aqueous polyurethane dispersions can be employed in waterborne coatings. In comparison to acrylics, polyurethanes are known to have better mechanical properties such as abrasion resistance, scratch and mar resistance, film toughness, and better solvent resistance. The polyurethane systems, however, are more costly due to the cost of the urethane raw material.
To optimize cost/performance benefits, coatings formulators have employed combinations of aqueous polyurethane and acrylic dispersions, physically blended together. Such blended dispersions can provide enhanced performance as compared to all acrylics and cost benefits as compared to all polyurethanes, but there are some inherent limitations to blended systems. Many polyurethane/acrylic dispersions are incompatible, resulting in unstable blends which may phase separate, gel or discolor upon storage. Furthermore, the homogeneity of the polyurethane/acrylic blends is questionable, both in the wet state and most importantly in the final dried coating.
Instead of merely mixing the polyurethane and acrylic dispersions together in a post-polymerization stage, polyurethane/acrylic hybrid dispersions have been formed in situ, as described in U.S. Pat. Nos. 4,927,876; 4,954,559; 5,354,807; 4,888,383; 4,918,129; 5,011,881; 3,684,759; 3,705,164; 4,198,330; 5,141,983; 4,644,030; 4,318,833; and 4,730,021; 5,137,961; 5,623,016; and 5,571,861. In this approach, ethylenically unsaturated monomers are polymerized by means of free radical polymerization in the presence of a preformed polyurethane dispersion. Each dispersion particle is stabilized by the hydrophilic moieties of the polyurethane, usually anionic groups in the polyurethane backbone. The resulting polyurethane/acrylic hybrid dispersion comprises acrylic and urethane polymer chains which are intimately mixed at the molecular level within each dispersion particle.
There continues to be a need, however, for coating formulations prepared with polyurethane/acrylic hybrid dispersions which provide superior wet resistance to alkyd substrates and solvent resistance without the disadvantages of prior art coatings.